In a filter arranged in an exhaust passage of an internal combustion engine, a failure such as breakage, erosion or the like may occur due to heat deterioration and time degradation. If the failure of the filter occurs, an amount of particulate matter, which is not trapped by the filter but flows into the exhaust passage at the downstream side thereof, will increase. Accordingly, there has been proposed a technique in which a particulate matter (PM) sensor is arranged in the exhaust passage at the downstream side of the filter, so that the failure of the filter is detected based on an output value of the PM sensor.
In Patent Document 1, there is disclosed a PM sensor (fine particulate sensor) which is provided with at least two electrodes. This PM sensor detects an amount of PM deposition in the PM sensor from measured data of an electrical property such as an alternating current impedance between the electrodes.
In Patent Document 2, there is disclosed a technique that detects a failure of a PM trapper based on a ratio between a detected value of an input PM sensor arranged at the upstream side of the PM trapper, and a detected value of an output PM sensor arranged at the downstream side of the PM trapper. In addition, in this Patent Document 2, there is described that when an amount of intake air is less than a predetermined amount, the failure detection of the PM tapper is not carried out.